Thoughts on a Recent Mew Gull

While out in Colorado chasing the Ross’ Gull at Cherry Creek Reservoir, I had an opportunity to study an adult Mew Gull at close range amid a large flock of Ring-billed Gulls. This is exactly the scenario that we in the east face when trying to score a vagrant Mew.

The first thing that jumped out at me was just how difficult this bird was to pick out, even when sitting some 10 feet in front of me! One of the other birders present called out the Mew and pointed to the water’s edge directly in front of me. After a frustrating 40 or 50 seconds, the bird materialized:

When seen like this, the ID is pretty straightforward. Thinner, all yellow bill. Dark eye. Mantle darker than Ring-billed. Tertial crescent broader and more contrasting.

But as the bird shifted around, thing became more difficult. Here is another shot of the Mew (at rear) with a Ring-billed and California Gull in front:

Notice how the mantle is darker than the Ring-billed, similar in tone to the California. Also notice how the tertial crescent really jumps out. But then the sun comes out, and whoa…the difference in mantle coloration all but disappears!

But, the differences in the tertial crescents remains. Here’s some more shots in strong, late afternoon light. The difference is subtle, but it’s there.

Note how the Mew’s dark eye stands out in relation to the pale, staring eye of the Ring-billed.

As we head into the best 8 weeks for gulling in the east, I’ll be looking at the tertial crescents of loafing flocks of Ring-bills, hoping to pick out a Mew. Now, those damn juveniles…

One thing this bird shows fairly well is more of the even brownish wash to the head (and often breast). This is sort of akin to the general differences in adult winter Thayer’s Gull versus Herring Gull. It’s not true all the time, but when searching through thousands of gulls and you don’t have time to check each one, I usually search for birds with this pattern to look at more carefully.

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